1. Field of the Invention
The present invention relates to rare earth permanent magnets, and particularly relates to rare earth permanent magnets having a uniform structure. The rare earth permanent magnets according to the present invention are suitable for use in devices such as electronic apparatuses, motors and actuators for electrical devices, and synchronous motors which requires heat-resistance, position sensors for electrical devices and rotation sensors and the like.
2. Description of Related Art
2-17-type Sm—Co-based magnets, whose typical structure is, for example, Sm(CoFeCuT)7.5, wherein T is Zr, Ti or the like, have high magnetic characteristics, excellent temperature characteristics, and corrosion-resistance, and are widely utilized as well as NdFeB-based magnets.
2-17-type Sm—Co-based magnets show a magnetic domain wall pinning type coercivity mechanism (FIG. 1a), and is different from 1-5 type Sm—Co-based magnets and NdFeB-based magnets, which show a nucleation growth type coercivity mechanism (FIG. 1b). Domain wall pinning magnets are magnets in which the magnetic moment of one phase of two separated phases is pinned at a number of locations throughout the domain wall minutely deposited between the phases, and therefore it is not possible to move the domain wall without applying a magnetic field of a specific value or more, resulting in that a large coercive force can be achieved. Such a characteristic can be seen from an initial magnetization curve as in FIG. 1A. It shows an initial magnetization curve such that, magnetization (M) does not increase unless an external magnetic field (H) of a specific value or more is applied, and that when magnetization starts to increase, the magnetization rapidly approaches saturation.
As shown in the photograph of FIG. 2, 2-17-type Sm—Co-based magnets have microstructures separated with coherency into two phases of a Sm(CoCuFe)5 particle boundary phase, which is rich in Cu, and a Sm2(CoFeCu)17 phase, which is rich in Fe. Although the size of the microstructure varies depending on the composition, typically, the size of the 2-17 phase is from about several tens of nanometers to 300 nm, and the size of the 1-5 boundary phase that separates the 2-17 phase is generally 10 nm or less. From observation of the magnet with a Lorentz electron microscope (Lorentz TEM), it is said that domain walls are present in the 1-5 phase.
From the result of this observation, and the fact that there is a difference in domain wall energy between the 1-5 phase and the 2-17 phase, it is said that the domain wall is pinned to the 1-5 phase due to the difference in domain wall energy of the 1-5 phase and the 2-17 phase. Generally, the following formula is used to estimate the size of the coercive force Hci.Hci=(γ2-17−γ1-5)/Msδwherein γ is domain wall energy, Ms is saturation magnetization of the domain wall portion, and δ is width of the domain wall.
The pinning of the domain wall cannot be released, unless an external magnetic field having a value corresponding to the difference between the domain energies is applied. This corresponds to the coercive force. Consequently, with conventional understanding, it was said that a separated structure, non-uniform structure or deposition of impurities which generates a difference in the domain wall energy or a non-uniformity in the domain wall energy is essential for a domain wall pinning coercivity mechanism, and that without these, coercive force could not be obtained. It was generally considered that in the 2-17-type Sm—Co-based magnets it is realized by two-phase separation of the 2-17 phase and the 1-5 phase.
However, as opposed to the above described general understanding on the pinning type coercive force, although Sm(CoCu)5, Ce(CoCo)5 and Ce(CoFeCu)5 magnets show initial magnetization curves of pinning type characteristics similar to 2-17-type Sm—Co-based magnets, no clear two-phase separation structure has been observed in these magnets. In some observations even using a transmission electron microscope (TEM), a two-phase separation structure has not been found in these magnets.
With regard to this, Lectard et al. theorized that the domain wall pinning is caused by concentration fluctuations of 10 nm or less, in other words, a state in which Co rich Sm(CoCu)5 and Cu rich Sm(CoCu)5 fluctuate on a micro scale, and the two phase separated structure can not be observed because the crystal structures are the same and there is very little difference in the lattice constants (see E. Lectard, C. H. Allibert, J. Applied Physics, 75 (1994), 6277., which is herein incorporated by reference.). This theory with regard to pinning type coercive force does not consider two-phase separation structures as the source of coercive force. However, it considers the differences in domain wall energy due to the concentration fluctuations as the source of the pinning type coercive force, and fundamentally, it is the same as conventional understanding on the matter.